Starting to get CBT

I had my fifth CBT session on Tuesday and I was told I have just three more to go before my course of therapy comes to an end. I think I am finally beginning to understand what it's all about, but I do still doubt whether it is the answer to everything.  It doesn't tackle the route causes of my anxiety and depression. They are still there. It does make me analyse problems and when I do worry, CBT makes me try to understand why I worry and how this gets worse through not doing anything to combat it.
It's easier said than done. I still fear the post. It's irrational I know but the arrival of the post brings on palpitations and sweating. I will not open it. Certain emails also have this effect and I have to ask my wife to read them for me. Utterly ridiculous I know.
CBT isn't the answer to everything. I have learnt how to understand better why I worry and how my anxiety can spiral out of control causing a severe bout of depression if not tackled. I'm aware of this. But applying the CBT methodology to every worry and fear is not possible.
I think I'm to begin focusing on assertiveness in the next session. That will be interesting. My years in middle and senior management taught me all I needed to know. But my breakdown three years ago took a lot from me including any degree of assertiveness I once had. In my view, you can only be assertive if you feel strong, and that's something I haven't been for a very long time.